204 ME. A. n. MICHAEL ON Aif [Mar. 5, 



is set with the point upwai'd ; they therefore form the levator 

 muscles of the mandible. The sigmoid piece is attached, by a 

 strong ligament (fig. 21, L.) which rises from a projection near the 

 middle of its anterior edge, to the chitinous bridge or shelf 

 (fig. 21, B.) on which the mandibles rest; thus the action of the 

 fan-shaped muscles cannot pull the sigmoid piece out of position. 



Formiug a fulcrum for the mandibles and a point of attachment 

 for their muscles is not the only office of the sigmoid pieces ; each 

 is hollow and its interior is an air-chamber (ac.) ; it is lined by a 

 thin loose membrane, which is in fact a continuation of the main 

 tracheal trunk from the stigma ; thus the air-chamber apparently 

 varies in shape according to the amount of air which it contains 

 for the moment : it, however, really occupies almost the whole 

 interior of the sigmoid piece, and is broadest about the middle ; 

 it does not, however, extend to the upper (anterior) end and 

 it diminishes to a very fine tube in the lower (posterior) curve of 

 the S and ends blindly some distance before the point of this 

 portion of the chitin. 



There are two passages out of this air-chamber, one is situated 

 in the posterior edge of the sigmoid piece near to but not at its 

 upper (anterior) end ; through this opening passes the principal 

 tracheal trunk (fig. 21, tra.) which runs to the stigma {S.). This 

 trunk is the only trachea in the body in which I have been able 

 to detect any ringing, but here it is very distinct ; it at first 

 rises between the mandibles, but when it has reached their upper 

 edge it turns for\^ard, and runs above and parallel to the mandible 

 on its own side for about one-fourth of the length of that organ ; 

 it then enlarges to a small bulb, pointed anteriorly, which contains 

 the stigma {S.). 



The second passage {tre.) out of the air-chamber is smaller and 

 is placed in the upper edge of the projection from the middle of 

 the anterior edge of the sigmoid piece to which, as before stated, 

 the ligament (Z.) is attached ; a small branch of the air-chamber 

 leads to this opening ; out of which passes the efferent tracheal 

 trunk, which shortly divides dichotomously, sending one trunk 

 forward and one backward, which almost immediately breaks up 

 into a multitude of extremely fine and delicate tracheae which 

 supply the body. 



In addition to an arrangement practically almost similar to this 

 Schaub describes and figures (see bis Taf. iii. fig. 8, tr.) a number of 

 very fine tracheae passing direct into the air-chamber through the 

 chitin of the sigmoid piece, and not springing from any tracheal 

 trunk. I cannot say what there may be in Hydrodroma, but I 

 can say with some confidence that nothing of the kind exists in 

 Thyas petrophiliis. I have several times obtained precisely the 

 appearance figured by Schaub ; but this, in the species I am 

 describing, has certainly arisen from the fan of muscles being torn 

 or cut away, leaving the numerous tendons by which they were 

 attached behind them ; these tendons part from the muscle much 

 more readily than from the sigmoid piece, 



